Due to the nature of electronics and mechanics, MEMS-based accelerometers are prone to having bias (offset) and sensitivity errors. These errors may drift and or change due to temperature, humidity, assembly stress and other changes in peripheral conditions. The inaccurate bias causes problems in sensor fusion, attitude (pitch, roll, and yaw) estimation, heading reference and other parameters that are dependent on the precision of the sensors' outputs. Specifically, if it is required to integrate the raw data, for instance, from acceleration to velocity or from angular rate to angle, the bias drift problem is significantly magnified.
A conventional solution is to calibrate each sensor for each axis for each unit in the factory line. However, this solution adds additional cost because each sensor on the unit has to be calibrated individually. The calibration might also need to change over time due to various reasons such as temperature.
For accelerometers, the conventional solution of calibration is to put the sensor board on a flat, absolutely horizontal table, measure the output of the unit facing upward when it is absolutely still, and then measure it again when facing downward under the same non-movement condition. This is repeated for three different axes. However, in practice, any deviation from these assumptions (flat surface, horizontal table, absolutely no movement) will deteriorate the accuracy of the calibration. In addition, this calibration method is only suitable for lab use or factory calibration. The end user cannot put the portable device, such as a smartphone, tablet, or compatible device on a flat, horizontal table with no movement.
For both MEMS-based accelerometers, the bias drift can be a large percentage of the initial value over the lifetime of the sensor. They vary from the factory calibrated values significantly as time passes by, or as their ambient condition changes, such as temperature.
Accordingly, it is desired is to provide systems and methods for in-use calibration of accelerometers.